1. Field of the Invention
The present invention relates to a modulator and a demodulator used in Orthogonal Frequency Division Multiplex (hereinafter referred to as OFDM) transmission, and more particularly to a phase correcting technique.
2. Description of the Background Art
Recently, a transmission system using the OFDM technique is attracting attention in digital audio broadcasting for mobile devices and terrestrial digital television broadcasting. The OFDM technique is a kind of multi-carrier modulation system, in which data to be transmitted (transmitted data) is assigned to a large number of subcarriers so arranged that adjacent subcarriers are orthogonal to each other and converted into a digital modulated signal in the time domain by inverse Fourier transform to generate an OFDM signal. In the OFDM transmission, the OFDM signal generated by applying the above-described processing to the transmitted data on the transmitting end is transmitted to the receiving end. The receiving end applies the reverse process to that applied on the transmitting end to the transmitted OFDM signal to reproduce the transmitted data. Since each data divided onto the subcarriers has a longer period, the OFDM signal is not susceptible to effects of delayed waves such as multipath.
The OFDM demodulation is achieved by applying Fourier transform by using a fast Fourier transform (hereinafter referred to as FFT) circuit to the OFDM signal down converted to the baseband in a quadrature detector circuit. At this time, in the quadrature detector circuit, frequency synchronization must be accurately established between the transmitter and receiver, and in FFT, one symbol section must correctly be taken in from the received OFDM signal with a given clock, to obtain the phase and amplitude information of the subcarriers through the Fourier transform.
When a frequency error occurs in the OFDM signal on the transmitting and receiving ends, or when a timing error occurs and one symbol section cannot accurately be captured, the subcarriers suffer phase rotation and then the transmitted data cannot be reproduced. In this way, the OFDM demodulation requires accurate frequency synchronization, symbol synchronization, and clock synchronization, and a conventional OFDM demodulator must establish the frequency synchronization, symbol synchronization, and clock synchronization by using synchronizing symbols. Accordingly, as shown in FIG. 11, in the OFDM signal So, a transmission frame is composed of a plurality of OFDM symbols OS each including a plurality of (. . . , k, k+1, k+2, . . . ) subcarriers SC, which is transmitted with a synchronizing symbol inserted in each frame. In the drawing, the vertical axis shows the phase φ and the horizontal axis shows the subcarrier frequency F. The character Δφ denotes a phase error between adjacent subcarrier.
As shown in FIG. 12, a null symbol is inserted as the synchronizing symbol RS in the beginning of n (n is an integer of one or larger) OFDM symbols OS to form one transmission frame Fr, and the null symbols are successively detected to establish synchronization. A signal from which synchronizing information can easily be obtained can be used as the null symbol, and it does not necessarily have to be an OFDM modulated signal. That is to say, since the OFDM signal So presents a waveform like random noise, it is difficult to obtain the synchronizing information directly from the time-domain waveform. Accordingly, a sine-waveform signal can be used for the frequency synchronization, and a waveform signal modulated by an amplitude shift keying (ASK) scheme, from which clock components can be easily extracted, can be used for the clock synchronization.
Referring to FIG. 13, the concept of the OFDM signal thus structured will be described. The left half of the diagram schematically shows the state SF of the OFDM signal in the frequency domain and the right half shows the state ST in the time domain. In the frequency-domain signal SF, a large number of subcarriers SC are orthogonally arranged on the frequency axis F in each OFDM symbol OS1 to OSn. This frequency-domain signal SF is subjected to inverse Fourier transform and OFDM modulation to generate the time-domain signal ST. The subcarriers SC are arranged at intervals P=1/PS (Hz) primarily modulated on the transmitting end, which are transformed by inverse Fourier transform to the signal ST having the symbol periods PS (sec) on the time axis T.
The transmitting end forms an OFDM signal transmission frame with the OFDM symbols OS and synchronization reference symbol RS and sends the frame. The synchronization reference symbol RS does not necessarily have to be an OFDM modulated symbol, but may be a signal having a waveform easy to use in synchronizing processing.
On the receiving end, only the synchronization reference symbol RS is taken out from the input signal ST on the time axis for synchronizing control. The OFDM symbols OS are cut out for each symbol period PS and converted by Fourier transform into the signal SF on the frequency axis, and thus the OFDM symbol OS is separated into the subcarriers SC. Subsequently, primary demodulation (data demodulation) is applied to the separated subcarriers to obtain the received data or to reproduce the transmitted data. In order to accurately maintain the synchronization in such OFDM signal demodulating process, the synchronization reference symbols must be transmitted periodically.
Referring to FIG. 14, the OFDM demodulator disclosed in Japanese Patent Laying-Open No. 8-102769 will now be described as an example of such a conventional OFDM demodulator. The OFDM demodulator DMC includes an A/D converter 101, a clock synchronization establishing portion 102, a quadrature detector 103, a frequency synchronization establishing unit 104, a Fast Fourier Transform unit (FFT) 105, a symbol synchronization establishing unit 106, and a primary demodulator 107. The OFDM signal So′ sent from the transmitter is supplied to the AID converter 101, clock synchronization establishing unit 102, frequency synchronization establishing unit 104, and symbol synchronization establishing unit 106.
The clock synchronization establishing unit 102 detects a synchronization error in a sampling clock between the transmitter and receiver in the OFDM signal on the basis of the synchronizing symbols RS in the OFDM signal So′. The clock synchronization establishing unit 102 then corrects the detected synchronization error to generate a synchronized sampling clock signal Ssc and outputs the same to the A/D converter 101. The A/D converter 101 converts the analogue OFDM signal So′ to a digital OFDM signal So synchronized in sampling clock component on the basis of the sampling clock signal Ssc and outputs the signal So to the quadrature detector 103.
The frequency synchronization establishing unit 104 detects a synchronization error in carrier signal frequency between the transmitter and receiver on the basis of the synchronizing symbol RS in the OFDM signal So′ and generates and outputs a synchronized frequency signal Scf to the quadrature detector 103. The quadrature detector 103 subjects the OFDM symbol OS (subcarriers SC) to quadrature detection in the digital OFDM signal So synchronized in sampling clock component on the basis of the frequency signal Scf, converts it from the intermediate frequency band into the OFDM signal Sb in the baseband, and outputs the signal Sb to the fast Fourier transform unit 105. Needless to say, the OFDM signal Sb in the baseband is synchronized in carrier signal frequency component and also synchronized in sampling clock component.
The symbol synchronization establishing unit 106 detects a synchronization error in symbol time window between the transmitter and receiver on the basis of the synchronizing symbol RS in the OFDM signal So′ to generate a synchronized symbol time window signal Sst and outputs the signal Sst to the fast Fourier transform unit 105. The fast Fourier transform unit 105 applies fast Fourier transform to the OFDM signal Sb in the baseband on the basis of the symbol time window signal Sst. The fast Fourier transform unit 105 separates the signal in the time domain to the subcarriers SC in the frequency domain for each OFDM symbol OS to generate a symbol synchronized subcarrier signal Sc and outputs the signal Sc to the primary demodulator 107. This subcarrier signal Sc is synchronized in symbol window and also synchronized in sampling clock and carrier signal frequency.
The primary demodulator 107 demodulates the subcarrier signal Sc outputted from the fast Fourier transform unit 105 for each subcarrier to reproduce the transmitted data Sd.
The conventional OFDM demodulator DMC applies Fourier transform by using the fast Fourier transform (FFT) unit 105 to the OFDM signal Sb down converted to the baseband by the quadrature detector circuit 103. At this time, the quadrature detector circuit 103 requires accurate frequency synchronization between transmitter and receiver, and the FFT accurately captures one symbol section from the received OFDM signal with a defined clock to obtain the phase and amplitude information of the subcarriers by Fourier transform.
In the data processing in the OFDM demodulator, the same conditions as those in the transmitter must be correctly reproduced about the sampling clock, carrier frequency, and FFT symbol window time. That is to say, in OFDM demodulation, synchronization must be established about the sampling clock, carrier frequency, and symbol time window. The conventional OFDM demodulator establishes the symbol synchronization and clock synchronization by detecting the synchronizing symbols intermittently inserted at certain intervals. In this case, the synchronizing symbols for several frames must be detected before establishing synchronization, and the OFDM symbols in this period cannot be correctly demodulated. However, the synchronization errors between the transmitter and receiver easily occur due to variations in the transmission environment and the like, which cause clock error, frequency error, and time window error. When such errors are occurring, the carriers of the OFDM modulated symbols suffer phase rotation of a amount corresponding to the errors with respect to the phases given at the time of transmission. Since information (transmitted data) are assigned to phases of the subcarriers, the transmitted data will be erroneously reproduced.
For these errors, the information is detected from the synchronization reference symbols, and the sampling clock error, carrier frequency error, and symbol time window error are fed back as the respective error signals to make adjustments for synchronization (to establish synchronization). Hence, since the subcarriers of OFDM symbols demodulated when the errors are occurring have phase rotation errors, the transmitted data are erroneously reproduced. Further, when the synchronizing symbols cannot be continuously detected at given intervals, stable synchronization cannot be established, and then it is very difficult to correctly demodulate the OFDM symbols transmitted in a burst manner.